That "Ultimate Computer" engine room has to be in the primary hull because of the long curving hallway walk to the engine room and the same walk leaving the engine room."

No, it does not have to be in the primary / saucer hull which is exactly the point these WIP deck plans try to illustrate and I addressed the circular corridor radius issue in the introduction.

If the radius of the corridor is taken literally, then the Ultimate Computer Engine Room either needs to be in the saucer, or be situated in an Enterprise larger than 947' (even 1080' barely squeezes it in) or tweak the structure of the corridor based on what was actually seen in the episode (I proposed something using a slight pivot here)

If the radius of the corridor is "squinted at" then, as per Robert_Comsol's plans the ship can accommodate it, although it does generate a secondary hull with quite a lot of corridor space...

As I mentioned in my post about the travel times above, we never see the start of the journey in Ultimate Computer so it might actually be longer, allowing for sufficient transit time down the secondary hull.

Personally, I lean to it being another room belonging to the Engineering section but one that is not used for propulsion control (after all, why should all Engine Rooms be solely used for this purpose? Starfleet may favour the layout for it's multiple disciple design, much as the producers of the show did).

I see it this way: In U.C, Kirk and gang travel to a part of the saucer where M-5 has been installed - an Engine Room, but one at the FRONT of the ship, behind those 3 round thingies which (in this argument) are the saucer deflector beam emitters. They are naturally part of the vessel's navigation systems, which is ideal since M-5 requires access to those circuits also. Deflector systems of course require a lot of power, and it is one of those main lines that M-5 taps into (much of the dismay of that poor engineer!)

Robert Comsol wrote:

But because of the visible corridor walk the engine room couldn't be in the saucer where it would block turbo lift travel to the engineering hull. I firmly believe Matt Jefferies anticipated this problem and therefore gave us an (impulse) engine room arrangement (Season One studio set) to circumvent this problem which you did so beautifully illustrate: http://s757.beta.photobucket.com/use...59794199719881

Of course with my layout decribed above there is no need to give up these two engine rooms at all!
The ease at which the S1 engine room set mirrors itself, allowing an 8' gap inbetween is a piece of design which impresses me as much today as when I first saw it - it really would be shame not to make use of it. I would also like to use full-length "cathedral tubes" but unfortunately the directors of different episodes couldn't resist shooting the set from the wrong angles, so bang goes that forced perspective illusion!!!